FIG. 10 schematically shows a cross sectional structure for explaining an example of an arrangement of a previously proposed GMR (Giant Magneto-Resistance) element making use of a GMR effect. For example, on a silicon substrate 100, a lower electrode 101, a lower ferromagnetic layer 103 (with a thickness of approximately 40 nm and a diameter of approximately 100 nm) made of a material such as Co, a nonmagnetic metal layer 104 (with a thickness of approximately 6 nm and a diameter of approximately 100 nm), an upper ferromagnetic layer 105 (with a thickness of approximately 2.5 nm and a diameter of approximately 100 nm) made of a material such as Co, and an upper electrode 106 are formed in this order. Further, a bit line 102 is formed on the lower electrode 101. It is known that such a GMR structure element can reverse the direction of magnetization of the upper ferromagnetic layer 105 by spin current injection from the upper electrode 106 side, that is, injection of electrons with polarized spins from the lower electrode 101 side. See for example JP-A-2004-207707 and J. A. Katine, et al., Current-Driven Magnetization Reversal and Spin-Wave Excitations in Co/Cu/Co Pillars, Physical Review Letters, Vol. 84, No. 14, pp. 3149-3152 (2000).
The operation principle of the element is explained as follows. First, a magnetic field with a sufficient strength is applied to the element to align the directions of magnetization of the lower ferromagnetic layer 103 and the upper ferromagnetic layer 105 in the same direction. FIG. 11A schematically shows a cross sectional view of the element shown in FIG. 10 in which the directions of magnetization in the ferromagnetic layers are aligned rightwardly (arrows in the figure showing the direction of magnetization) in each ferromagnetic layer. In the following drawings, arrows have the same meaning. The state is to be referred to as the parallel state (P-state). In this state, an electric current flowing in the direction from the lower electrode 101 side to the upper electrode 106 side causes electrons to be injected from the upper electrode 106 to the upper ferromagnetic layer 105. In the upper electrode 106, the electron spins are in a state in which the distribution of up-spins matches that of down-spins. In the upper ferromagnetic layer 105, however, due to interaction (s-d interaction) between the electron spins and the spins of ferromagnetic metal atoms, spins of conduction electrons are polarized so that the spins in parallel with the direction of magnetization of the upper ferromagnetic layer 105 become the majority. This is referred to as polarization of spin. However, the upper ferromagnetic layer 105 of the layered films now being considered is thin, so that the polarization remains slight. When the conduction electrons with thus slightly polarized spins pass through the nonmagnetic metal layer 104 to reach the surface of the lower ferromagnetic layer 103, electrons having spins in parallel with the direction of magnetization of the lower ferromagnetic layer 103 are injected into the lower ferromagnetic layer 103. However, electrons having spins in the directions opposite to the direction of the magnetization of the lower ferromagnetic layer 103 are reflected to be injected into the upper ferromagnetic layer 105 again. The lower ferromagnetic layer 103, being thick, functions as a spin filter that gives priority to pass electrons having spins in the direction in parallel with the direction of the magnetization of the lower ferromagnetic layer 103 itself. As a result, the majority carriers in the upper ferromagnetic layer 105 become electrons having spins in the direction opposite to the direction of the magnetization of the lower ferromagnetic layer 103. Each of thus given electrons exerts a torque on the magnetization of the upper ferromagnetic layer 105 in the opposite direction to reverse the direction of magnetization thereof. Current exceeding a certain critical level causes the direction of the magnetization of the upper ferromagnetic layer 105 to rotate by the exerted torque, by which the state with the directions of magnetization of the upper ferromagnetic layer 105 and the lower ferromagnetic layer 103 changes from the P-state shown in FIG. 11A to the anti-parallel state (AP state) shown in FIG. 11B.
An explanation about the case in which current flows from the upper electrode 106 to the lower electrode 101 in the element in the AP-state now follows. In this case, electrons are injected from the lower electrode 101 to the lower ferromagnetic layer 103. Also, in the lower electrode 101, the electron spins are in the state in which the distribution of up-spins matches that of down-spins. In the ferromagnetic layers, however, there is interaction (s-d interaction) between the electron spins and the spins of ferromagnetic metal atoms. Here, the thick lower ferromagnetic layer 103 causes spins of conduction electrons to be polarized so that the spins in parallel with the direction of magnetization of the upper ferromagnetic layer 105 become the majority. When the conduction electrons with largely polarized spins pass through the nonmagnetic metal layer 104 to reach the surface of the upper ferromagnetic layer 105, the majority of electrons having spins in antiparallel with the direction of magnetization of the upper ferromagnetic layer 105 are injected into the upper ferromagnetic layer 105. As a result, each of the injected electrons, having spins in the directions in parallel with the direction of the magnetization of the lower ferromagnetic layer 103, exerts a torque on the magnetization of the upper ferromagnetic layer 105 in the opposite direction to reverse the direction of magnetization thereof. Current exceeding a certain critical level causes the direction of the magnetization of the upper ferromagnetic layer 105 to rotate by the exerted torque, by which the state with the directions of magnetization of the upper ferromagnetic layer 105 and the lower ferromagnetic layer 103 returns from the AP-state shown in FIG. 11B to the P state shown in FIG. 11A.
The electric resistance of a GMR element is known to be small in the P-state and large in the AP-state with the rate of change being several tens of percent. By using the GMR effect, a reading head can be manufactured for a hard disk. FIG. 12 is a schematic view showing a planar structure of an MRAM (Magnetic Random Access Memory) in which a plurality of the GMR elements shown in FIG. 10 are connected to use the inversion of magnetization of GMR elements by current injection. With the use of the arrangement as shown in FIG. 12, writing (inversion of magnetization) and reading out (detection of electric resistance values corresponding to states of magnetization of recording cells 109) of bit information to and from the recording cells 109 are principally possible by a group of laterally running word lines 108 and a group of longitudinally running bit lines 107.
FIGS. 13, 14A, and 14B are schematic cross sectional views each for explaining a phenomenon of displacement of a magnetic domain wall formed in a ferromagnetic wire in a related magnetic domain wall displacement element by a current flowing in the ferromagnetic wire. See for example A. Yamaguchi, et al., Real-Space Observation of Current-Driven Domain Wall Motion in Submicron Magnetic Wires, Physical Review Letters, Vol. 92 No. 7, 077205 (2004). FIG. 13 is a schematic cross sectional view showing an arrangement of the element, in which a ferromagnetic layer 121 (10 nm in thickness and several micrometers in length) is formed on an insulator substrate 120. On the ferromagnetic layer 121, a left electrode 122 and a right electrode 123 are formed. For the ferromagnetic layer 121, a material such as a permalloy (Ni81Fe19) thin film is used. For the left and right electrodes 122 and 123, a material such as copper (Cu), gold (Au), or platinum (Pt) is used. FIGS. 14A and 14B are schematic cross sectional views for explaining the principle of displacement of a magnetic domain wall 124 when current flows between the left electrode 122 and the right electrode 123. In each of the views, the directions of magnetization in the magnetic layer are shown with arrows like in the above explanation.
First, as shown in FIG. 14A, consider the case in which there is one magnetic domain wall 124 in the region of the ferromagnetic layer 121 between two electrodes and the direction of magnetization on the right side of the magnetic domain wall 124 is opposite to the direction of magnetization on the left side. When flowing current in this state from the right electrode 123 to the left electrode 122, the current crosses the magnetic domain wall 124. At this time, electrons are injected from the left electrode 122 into the ferromagnetic layer 121 to flow into the right electrode 123. The directions of spins of electrons injected into the ferromagnetic layer 121 are considered to be aligned by the s-d interaction in the same direction as the direction of magnetization in the region on the left side of the magnetic domain wall 124 in the ferromagnetic layer 121 (also referred to as polarization). The magnetization due to spins of the polarized electrons is taken as SI (a rightward vector). Then, when the spin-polarized electrons pass through the magnetic domain wall 124 and are injected into the region on the right-hand side of the magnetic domain wall 124 of the ferromagnetic layer 121, the direction of spins of electrons is aligned this time by the s-d interaction in the same direction as the direction of magnetization opposite to the direction before the electrons pass through the magnetic domain wall 124. The magnetization due to spins of the electrons polarized on the right-hand side of the magnetic domain wall 124 is taken as Sr (a leftward vector). Moreover, the magnetization on the left-hand side of the ferromagnetic layer 124 and the magnetization on the right-hand side are taken as Ml (a rightward vector) and Mr (a leftward vector), respectively.
With the direction of Sl supposed to be positive, in the process in which electrons move from the left-hand side to the right-hand side of the magnetic domain wall 124, the magnetization Sl due to electron spin changes to Sr, resulting in an increase in electron spins in the negative direction. Before and after electrons cross the magnetic domain wall, the total sum (Ml+Sl+Mr+Sr) of magnetization of the magnetic material and spin angular momentum of conduction electrons is conserved to be constant. In a process in which conduction electrons on the left-hand side of the magnetic domain wall cross the magnetic domain wall, the total sum of whole spin angular momentum of electrons (Sl+Sr) increases by 2Sr (decreases by 2Sl). Since the total sum (Ml+Sl+Mr+Sr) of magnetization of the magnetic material and spin angular momentum of the conduction electrons is conserved, by the conduction electrons crossing the magnetic domain wall 124 from the left-hand side to the right-hand side, the total sum (Ml+Mr) of magnetization increases by 2Sl (decreases by 2Sr). In other words, by the conduction electrons crossing the magnetic domain wall 124 from the left-hand side to the right-hand side, the magnetization of the magnetic domain wall 124 is to go on increasing in the positive direction (in the direction of Ml). Namely, the magnetic domain wall 124 is to go on moving in the same direction as the direction in which electrons flow.
FIGS. 14A and 14B show the difference in position of the magnetic domain wall 124 between the state before current flows from the electrode 123 and the state after current flows from the right electrode 123. It is known that the magnetic domain wall 124 thus moves in the direction opposite to the direction in which current flows. It is reported that the current density enabling the displacement of the magnetic domain wall is of the order of 108 A/cm2 in the case of metallic magnetic material such as permalloy and of the order of 8×104 A/cm2 in the case of ferromagnetic semiconductor and that, by increasing the current density, the displacement speed of the magnetic domain wall becomes of the order of 3 m/s. See for example Yamaguchi's paper and Michihiko Yamanouchi, Abstract for 60th Annual Meeting Phys. Soc. Jpn., p. 27aYP-5, Mar. 27 (2005).
Each of the above-explained two technologies inverts the magnetization direction by flow current in the element. Its operation principle is based on the fact that, when spin-polarized electrons are injected into a ferromagnet, a torque due to electron spin is exerted on the magnetization of the ferromagnet. At this time, the total of the magnetization due to spins of the injected free electrons and the magnetization of the ferromagnet is conserved. Thus, for bringing about inversion of magnetization with a slight amount of injected electrons (or an injected current), the volume and the magnitude of saturation magnetization of the ferromagnet subjected to inversion of magnetization must be made small.
For example, in the case of the MRAM shown in FIG. 12, when its volume and its saturation magnetization are made small, a problem arises in that thermal stability of recording bit, namely thermal stability of magnetization of the recording cell 109, is reduced, causing thermal fluctuation of magnetization by thermal disturbances, even at room temperature, making it impossible to keep the magnetization of the recording cell. Also in the arrangement shown in FIG. 13, for carrying out high speed displacement of the magnetic domain wall by a slight current, saturation magnetization must be lowered. However, lowering the magnetization saturation increases thermal fluctuation of magnetization forming the magnetic domain wall. Thus, it can be easily supposed that a problem arises in that the position of the magnetic domain wall is randomly displaced by thermal agitation.
Furthermore, with the structure shown in FIG. 13, although it is possible to induce a change in the state of magnetization, i.e., displacement of the magnetic domain wall, by supplying current, it is difficult to detect a state of magnetization. This is because, in the case of the arrangement shown in FIG. 13, only the position of the magnetic domain wall changes without a change in the length of the magnetic layer in which current flows. Although the ratio of the length of the region magnetized rightward and the length of the region magnetized leftward changes in the ferromagnet 121, it is considered that the rightward resistivity and the leftward resistivity are the same. Therefore, the difference in the electric resistance due to change in the ratio of the lengths is in a negligible level. Hence, only with such displacement of the magnetic domain wall, there is no large change in the electric resistance between both of the electrodes.
Accordingly, there remains a need for an element in which its magnetized state can be changed by flowing current between two electrodes thereof and changing the electric resistance between the two electrodes depending on its magnetized state, to improve the thermal stability, while the critical current necessary for changing the magnetized state remains small. Also, there remains a need for an element in which its magnetic domain wall is displaceable by flowing current between two electrodes of a magnetic material so that the electric resistance between the two electrodes is changed. The present invention addresses this need.